The present invention relates in general to polymer matrices, and in particular to a new and useful method of dispersing fine particles of different incompatible polymers into one polymer matrix.
ABS is an important thermoplastic having U.S. markets of several billion pounds per year with selling prices of $0.50 to $1.00 per pound. It is a two-phase polymer. The continuous or matrix phase is a copolymer of styrene and acrylonitrile typically formed by a free-radical polymerization and typically containing 25 to 35% acrylonitrile. By itself, this matrix phase would be hard and glossy when injection molded but would lack impact strength. Major improvements in practical toughness are possible when a polybutadiene rubber is incorporated into the plastic as a dispersed phase.
Several processes exist for incorporation of rubber into a continuous polymer matrix. Physical blends of rubber and the polymer are possible using intensive shear devices such as extruders, but these give a product of poor quality. A superior product is formed by the so-called emulsion process where fine particles of rubber are created by emulsification and hwere the matrix phase is formed by copolymerization of styrene and acrylonitrile in the presence of these particles. This process gives an impact-resistant product having good molded gloss. The average rubber particle size is typically less than one micron.
The emulsion process is relatively expensive, and substantial efforts have been devoted to replacing it with a cheaper, bulk process, analogous to bulk processes commonly used for the manufacture of impact polystyrene. In these bulk processes, a polybutadiene rubber is dissolved in the monomers which will ultimately be polymerized to form the matrix phase, these being styrene and acrylonitrile in the case of ABS.
The polymerization is initiated by heat or chemical means and takes place in an agitated vessel. The vessel can be operated as a batch reactor but continuous flow is more common in modern processes. As the polymer is made, it forms a separate phase from that of the rubber. The system consists of two liquid phases, one being rubber dissolved in the monomeric mixture and the other being newly formed copolymer dissolved in the monomeric mixture. A phase diagram for the system is illustrated in FIG. 1.
The feed to an agitated vessel consists of rubber and monomers and is in the single phase region at 10. The effluent stream from the agitated vessel contains rubber, monomers, and (co) polymers at 11 and is in the two phase region 12. Typically, the agitated vessel will be operated such that the polymer-rich phase has a substantially greater volume than the rubber-rich phase and will thus be the continuous phase. The rubber-rich phase will be distributed into more or less spherical droplets the size of which depends on operating conditions within the agitated vessel and noteably on the speed of the agitator. In practice, rubber particle sizes of 2 to 5 microns are achieved by this process. Particles of 1 micron or smaller are difficult or impossible to achieve even with very high levels of agitation.
Rubber particle sizes in the range of 2 to 5 microns are suitable for most grades of impact polystyrene but are unsuitable for ABS for which good molded gloss in a product requirement. This fact has lead to the abondonment of bulk processes for molding grades of ABS or to the creation of hybrid processes in which small rubber particles are achieved through emulsification.